An empirical study of the lidar ratio and its variability, with implications for determining climate forcing by satellite-borne lidar

Abstract:

It is well-established that atmospheric aerosols play a role in determining the radiative balance of the Earth. Further, it is believed that anthropogenic aerosols may be causing significant cooling at the Earth's surface (the "aerosol direct effect"). Several methods have been used to quantify this effect, but the uncertainties in aerosol radiative forcing are still unacceptably high. One method that holds particular promise for reducing these uncertainties is laser radar, or "lidar". Lidar is analogous to radar but it operates at visible wavelengths, sensing range-resolved backscatter from clouds and aerosols. A satellite-based lidar could make vertically-resolved measurements of aerosols with global coverage.The single greatest obstacle to using lidar to determine aerosol radiative forcing is the conversion of the elastically-scattered lidar return signal to aerosol light extinction. To do so one must know the extinction-to-180° backscatter ratio, or "lidar ratio" of atmospheric aerosols. Presented herein is a method for empirically determining the lidar ratio, S, by measuring each of its components with in-situ instruments. A new instrument, the 180° backscatter nephelometer, was invented for this purpose and its design, calibration and performance are discussed.The mean, variability, and uncertainty of the lidar ratio for a broad range of atmospheric aerosols as measured at four field experiments are presented. For one experiment, in-situ and Raman lidar-derived values of light extinction and 180° backscatter are compared. The Raman lidar values are, on average, higher by ∼30%. Several possibilities for this bias are discussed.The in-situ measurements indicate that the lidar ratio is well-constrained to ∼30sr for a clean marine aerosol for a relative humidity of 50--80%; below 50% RH the lidar ratio drops to ∼15. The lidar ratio of anthropogenically influenced aerosol is much more variable, ranging from about 25 to 90sr. However, S is more well-constrained within air masses from a common region-of-origin and with a similar water content. The aerosol size distribution and relative humidity are controlling factors for the lidar ratio, but the relationship between S and these factors is quite variable. Finally, the lidar ratio does appear to be more well-constrained at very high values of light extinction.